PROJECT SUMMARY/ABSTRACT Epilepsy affects 3.4 million people in the US, with medial temporal lobe epilepsy (MTLE) the most common type. The dentate gyrus (DG) is central to the pathophysiology of MTLE. The DG becomes hyperexcitable in MTLE due to the loss of DG inhibitory interneurons and an increase in the number of recurrent synapses of DG granule cells (GCs) resulting positive excitatory feedback. While activation of DG inhibitory interneurons can have an anti-seizure effect, these are lost in MTLE and driven by GCs, urging us to examine an alternative mechanism to control the DG. Surprisingly, the DG has few inputs. The entorhinal area conveys information from the cerebral cortex and is excitatory. The septal area and SuM are similarly large inputs, but the septal area acts indirectly on GCs by way of inhibitory interneurons that are lost in MTLE. The SuM, by contrast, directly innervates GCs, including with highly unusual neurons that release both GABA (an inhibitory neurotransmitter) and glutamate (an excitatory neurotransmitter). The role of these neurons is unclear, and, unlike other SuM neuronal types, these GABA/glutamate neurons do not promote wakefulness or theta EEG activity. Preliminary data reveals that disruption of GABA release from these neurons results in high mortality in mice that were not monitored with video/EEG, and a low-voltage abnormal EEG in intermittently monitored mice, both consistent with established status epilepticus. The central hypothesis of the proposed studies is that these SuM GABA/glutamate neurons stabilize the DG and provide a promising selective target for the treatment of MTLE. Aim I will delineate the input and output relations of SuM neuronal groups, helping to understand the circuit basis for SuM effects and SuM- hippocampal interactions. Aim II will examine the effects of disruption of GABA release from SuM GABA/glutamate neurons (using the Cre/lox technique), chronically recording video/EEG and hippocampal field potentials, with the hypothesis that the mice will develop spontaneous seizures arising in the hippocampus, and progress to status epilepticus. Aim III will examine the effects of chemogenetic activation of SuM GABA/glutamate neurons on the frequency and severity of spontaneous seizures in the intra-amygdala kainate model of MTLE. Overall, these studies will provide an improved understanding of SuM-DG interaction in normal physiology and MTLE, providing a potentially specific modulatory target for the treatment of MTLE. These studies will provide key training and career development experiences enabling the applicant to reach the career development goal of becoming a rigorous and successful independent physician-scientist studying the systems neuroscience of epilepsy. The mentorship team has well-established expertise in mentoring junior faculty, and will provide training in epilepsy basic science, neuroanatomy, electrophysiology, vectors and chemogenetics, as well as in the preparation of grants and manuscripts. The Career Development plan includes training in the responsible conduct of research, statistics, design and reproducibility, signal processing, as well as conference attendance enabling career development training and the communication of scientific work.